Right now, there are completed movies sitting on studio shelves, yet to be released. Why? Because it's not the right season.
Hollywood runs its release schedule on a season-by-season setup that's predicated on longstanding assumptions about who will be most likely to spend free time at a theater and when. This doesn't just decide when things come out; because space is finite it also plays a role in deciding what gets made in the first place, and is also why certain film critics get into moods about slow release periods.
Understanding this can offer an insight into why your viewing options on a given weekend are what they are. Since we're currently in one of those aforementioned slow release periods, here's a quick and easy breakdown of how it basically works, courtesy moi.
Once upon a time, Summer wasn't that big of a deal for movie theaters or any indoor business. Outdoor drive-in theaters did brisk business, but because they were mainly being used as a teenage hookup excuse, the movies didn't have to be especially consequential (or expensive), and while younger children were out of school, they weren't expected to have their own money. "Kiddie Matinees" - day-length events comprised of low-budget family fare, cartoons, and so forth, were set up by individual theaters but weren't necessarily part of the big studio's revenue stream.
This slowly began to change in the mid-1950s with the advent of indoor air-conditioning, but it really took off in the 70s as the concept of "kids have money now" dawned on Hollywood and Star Wars ushered in the blockbuster era. Today, Summer is Hollywood's cash cow, the period when the most profitable demographics have the most time to attend the most movies. Studios trip all over one another to make space for what are now called "tentpoles" - moneymaking mega-productions that (literally) "hold up the tent" by covering the expense of everything else the studio wants to do.
Granted, there is such a thing as "counter-programming," wherein a movie that might seem more suited to another season gets released in the Summer on the premise that it might stand out, but that's been known to backfire as often as it works out. With rare exception, Summer is the season of big-budget genre movies and family fare; the only thing that's managed to change recently is that the season is actually getting longer. The first "Summer Movies" are now expected to bow in what is technically still late spring. The Avengers, expected to be one of the biggest Summer Movies of 2012, will bow in May, for example.
Fall is Awards Season, primarily because "Awards Movies" are presumed to be aimed mainly at folks over 35 and Fall is when they are presumed to have time outside of work/home that isn't taken up by the hustle of the winter holidays. It's also because the Oscar voting pool is kind of ancient and are expected to not remember anything they saw for more than four months, so films hoping to take home a Little Gold Man want to come out just before year's end.
This is when you get the bulk of your "prestige pictures" - films not necessarily expected to make big money but to be remembered as enduring conversation pieces down the line. This will also be the point where films that were only ever designed to impress attendees at film festivals earlier in the year will make their obligatory awards-qualifying theatrical runs, thanks to the widely-available schedule space; and when niche independents (self-financed bookings, religious films, agenda-backed political "documentaries") will get their shot.